Ultrasound detection based on phase shift

ABSTRACT

A system and method of detecting acoustic waves including directing a continuous-wave source laser beam to an optical resonator that is impinged by acoustic the waves. Optionally, the source laser beam can propagate through the optical resonator, thereby generating a propagated laser beam. Using an interferometer, the acoustic waves can be detected by monitoring transients in an optical phase of the propagated laser beam.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. patent application Ser. No.17/420,903, filed Jul. 6, 2021, which is a National Phase of PCT PatentApplication No. PCT/IL2020/050016 having International filing date ofJan. 7, 2020, which claims the benefit of priority of U.S. ProvisionalPatent Application Nos. 62/789,015, filed Jan. 7, 2019, and 62/833,912,filed Apr. 15, 2019, the contents of which are all incorporated hereinby reference in their entirety.

BACKGROUND

The invention relates to the field of ultrasound detection.

In the last decade, there has been an increasing interest in opticaltechniques for ultrasound detection, as an alternative to piezoelectricdevices, e.g., in biomedical applications. One of the common opticalapproaches for ultrasound detection is the use of optical resonators,which can trap light within small volumes and thus facilitate detectorminiaturization without loss of sensitivity. When an acoustic waveimpinges on an optical resonator, it perturbs its refractive index anddeforms its structure, resulting in a modulation of the resonancewavelength. By monitoring the shifts in the resonance wavelength, onecan effectively measure the ultrasound-induced pressure within theresonator. Despite the simplicity of this method, the maximum achievablesignal-to-noise ratio is limited by the laser phase and intensity noise.

The foregoing examples of the related art and limitations relatedtherewith are intended to be illustrative and not exclusive. Otherlimitations of the related art will become apparent to those of skill inthe art upon a reading of the specification and a study of the figures.

SUMMARY

The following embodiments and aspects thereof are described andillustrated in conjunction with systems, tools and methods which aremeant to be exemplary and illustrative, not limiting in scope.

There is provide, in an embodiment, a system comprising an opticalresonator configured to be impinged by acoustic waves; a continuous-wavesource laser beam directed to said optical resonator, wherein saidsource laser beam is propagated through said optical resonator, therebygenerating a propagated laser beam; and an interferometer configured fordetecting a signal associated with said acoustic wave by monitoringtransients in the optical phase in said propagated laser beam, whereinsaid transients are indicative of a waveform of said acoustic wave.

There is also provided, in an embodiment, a method comprising: directinga continuous-wave source laser beam to an optical resonator that isimpinged by acoustic waves, wherein said source laser beam is propagatedthrough said optical resonator, thereby generating a propagated laserbeam; and detecting a signal associated with said acoustic waves bymonitoring, using an interferometer, transients in the optical phase insaid propagated laser beam, wherein said transients are indicative of awaveform of said acoustic wave.

In some embodiments, said monitoring comprises interfering saidpropagated laser beam with a reference beam.

In some embodiments, said reference beam comprises a replica of saidsource laser beam. In some embodiments, said propagated laser beam andsaid reference laser beam have an optical path difference equal to zeroat a wavelength of said source laser beam.

In some embodiments, said reference beam comprises a time-delayedversion of said propagated laser beam.

In some embodiments, said optical resonator is selected from the groupconsisting of: π phase-shifted Bragg grating (π-BG), Fabry-Perot cavity,and optical-ring resonator.

In some embodiments, said laser beam is tuned to a center of atransmission notch of said optical resonator.

In some embodiments, said interferometer is a Mach-Zehnderinterferometer (MZI).

In some embodiments, said monitoring further comprises measuring anoptical power transmission in said propagated laser beam and saidreference laser beam, wherein said optical power transmission isindicative of said transients.

In some embodiments, said measuring of said optical power transmissionis performed by at least one balanced photo-detector.

In some embodiments, said acoustic waves are ultrasound acoustic waves.

In some embodiments, said ultrasound acoustic waves are generatedopto-acoustically via the transformation of a modulated optical beaminto acoustic waves.

In some embodiments, said modulated optical beam comprises opticalpulses.

In addition to the exemplary aspects and embodiments described above,further aspects and embodiments will become apparent by reference to thefigures and by study of the following detailed description.

BRIEF DESCRIPTION OF THE FIGURES

Exemplary embodiments are illustrated in referenced figures. Dimensionsof components and features shown in the figures are generally chosen forconvenience and clarity of presentation and are not necessarily shown toscale. The figures are listed below.

FIGS. 1A-1B illustrate an intensity resonator response function and aphase resonator response function;

FIGS. 2A-2B schematically illustrate exemplary systems for ultrasounddetection, in accordance with some embodiments;

FIG. 3 is a flowchart of the functional steps of a method for ultrasounddetection according, to an embodiment; and

FIGS. 4A-4C illustrate experimental results.

DETAILED DESCRIPTION

Disclosed herein are a system and method for ultrasound detection, basedon monitoring variations in the phase of a continuous-wave (CW) laserbeam applied to an optical detection device, which is subjected toultrasound acoustic waves.

In some embodiments, the optical detection device comprises an opticalresonator, wherein the ultrasound acoustic waves cause a modulation of arefractive index of the optical resonator, and wherein the modulation iscorrelated with a waveform of the ultrasound.

In some embodiments, an interrogation and/or detection light source,e.g., a continuous wave (CW) laser beam, is directed at the opticalresonator, wherein the optical resonator causes a modulation of thedetection light source which is correlated with the changes in therefractive index of the optical resonator.

In some embodiments, the optical detection device comprises aninterferometer which measures interference patterns between thedetection light source and, e.g., a reference light beam.

In some embodiments, the measurement comprises measuring a phase shiftbetween the sample and reference light sources, wherein the phase shiftis indicative of a waveform of the ultrasound waves.

In some embodiments, the present system provides for an improvement indetection sensitivity over current ultrasound detectors, based, at leastin part, on improvement in the signal-to-noise (SNR) ratio.

In some embodiments, the present system provides for an ultrasounddetection method using passive high-Q optical resonators, based on abalanced phase detection method that can significantly reduce the effectof the laser phase noise on the measurement, while maintaining the gainin signal due to the high Q of the resonator. In some embodiments, anexemplary system as described herein below may be capable of a 24-foldincrease in SNR compared to intensity-based methods, withshot-noise-limited detection for powers up to 1 mW.

For example, ultrasound detectors based on measuring an intensity of adetection light source (e.g., a laser beam) at the output of an opticalresonator, suffer from two potential noise sources: Photodetector noise,and laser frequency noise. Photodetector noise can be offset at leastpartially by, e.g., using higher laser powers or resonators with higherQ-factors. However, for sufficiently high powers and Q-factors, thelaser frequency noise becomes the dominant noise factor, for whichsensitivity cannot be further improved via higher Q factors or laserpowers. In this case, the optical SNR can be improved only by reducingthe frequency noise of the laser, i.e. using interrogation lasers withnarrower linewidths, leading to a higher system cost.

Other methods for improving SNR include, e.g., adding a second, staticresonator during the ultrasound measurement and effectively subtractingthe noise signal from the ultrasound signal, or via coherence-restoredpulse interferometry (CRPI). However, these approaches carry a cost of amore complex optical system, that may also require post-processing ofthe signals.

A potential advantage of the present system is, therefore, in that itprovides for a significantly reduced frequency noise compared tointensity-based detection system, without adding to the complexity andcost of existing detection systems. In addition, the present system isgenerally compatible with any passive optical resonator.

The transmission spectrum of many optical resonators, as a function ofwavelength, may be described as follows:

$\begin{matrix}{{t(k)} = \frac{\alpha}{\alpha - {i\Delta k}}} & (1)\end{matrix}$

where α is a constant and Δk is the wavenumber detuning, given by:

$\begin{matrix}{{\Delta k} = {{- 2}\pi{{n_{0}\left( {\frac{1}{\lambda} - \frac{1}{\lambda_{0}}} \right)}.}}} & (2)\end{matrix}$

Here, n₀ is the refractive index of the fiber, λ is the opticalwavelength, and λ₀ is the central wavelength of the grating.

The power transmission T(Δk) and phase ϕ(Δk) corresponding to t(Δk) arerespectively given by

$\begin{matrix}{{{❘{t\left( {\Delta k} \right)}❘}^{2} = \frac{\alpha^{2}}{\alpha^{2} + {\Delta k^{2}}}},} & (3)\end{matrix}$${{phi}\left( {\Delta k} \right)} = {{arc}\tan{\frac{\alpha}{\Delta k}.}}$

The power transmission T(Δk) has a Lorentzian shape with a full-width athalf maximum (FWHM) of 2α, yielding a Q factor given by Q=1/2λ₀.

When ultrasound detection is performed via intensity-based methods, thelaser source is often tuned to the steepest slope of T(Δk), obtained atΔk=±α/3 and equal to ∓0.54α⁻¹. Accordingly, when the resonance shifts byδk, the resulting change in the monitored power transmission isδT=∓0.54α⁻¹δk. Because δT is inversely proportional to α, and thus tothe Q factor, the sensitivity obtained via intensity-based methods isoften optimized by using resonators with high Q factors. However, notonly the maximum slope of T(k) is inversely proportional to a, but alsothe maximum slope of ϕ(Δk), which is obtained at Δk=0 and is equal to−α⁻¹. Thus, the gain in signal obtained in intensity-based measurementsusing high-Q resonators may also be obtained in the present,phase-based, system.

FIGS. 1A-1B illustrate an intensity resonator response function (FIG.1A) and a phase resonator response function (FIG. 1B), wherein thedashed lines represent the spectrum shift caused by ultrasoundexcitation. In some embodiments, and as shown in FIGS. 1A and 1B, Whenthe resonator interacts with the acoustic wave, stress and strain aregenerated within the structure, which produces a shift of the opticalspectrum.

FIGS. 2A-2B schematically illustrate exemplary systems 200 and 220 forultrasound detection, in accordance with some embodiments.

In some embodiments, system 200 in FIG. 2A comprises, e.g., an opticalresonator 202. In some embodiments, exemplary optical resonator 202 maycomprise, e.g., a π phase-shifted fiber-Bragg grating (π-FBG) with aresonance of λ_(B)=1549.11 nm, and a FWHM of approximately 2 pm. In someembodiments, the FWHM is approximately 0.5 GHz. In some embodiments, theoptical resonator may comprise any passive optical resonator, such as aFabry-Perot cavity, and optical ring or micro-ring resonators. In someembodiments, additional and/or other types of optical resonators may beused. In some embodiments, an optical resonator of system 200 may befabricated in silicon waveguide, silicon nitride, silica, and/or polymerwaveguides. In some embodiments, the optical resonator may be a part ofany circuit, device and/or arrangement which comprises an opticalresonator, such as an interferometer.

In some embodiments, exemplary system 200 may be configured as aninterferometer, e.g., a fiber-based Mach-Zehnder interferometer (MZI).For example, optical resonator 202 may be placed in one of the arms of afiber-based Mach-Zehnder interferometer (MZI), e.g., arm 1 in FIG. 1 .

A potential advantage of using fiber-Bragg gratings is in that there isno tradeoff between sensitivity and the effective sensing area, becauselight at resonance frequencies undergoes strong localization centered onthe phase shift, which allows achieving very small sensing lengths.Additionally, in some embodiments, fiber-Bragg gratings can be small insize, immune against electromagnetic interference, and mechanicallyflexible.

In some embodiments, the fiber-Bragg grating can be fabricated in apolarization-maintaining fiber. In some embodiments, the fiber-Bragggrating can be recoated with acrylic coating after fabrication.

System 200 may further comprise a source laser beam 204, which may be acontinuous wave (CW) laser beam source. In some embodiments, sourcelaser beam 204 may be tuned to the maximum transmission notch of opticalresonator 202 (Δk=0). In some embodiments, the source laser beam 204wavelength may be tuned to a resonant wavelength of the π-FBG. In someembodiments, for a source laser beam 204 comprising the resonantwavelength, the phase response of the π-FBG, is linear and/or comprisesmaximum sensitivity to ultrasound signals.

In some embodiments, source laser beam 204 may be divided into two parts(sample and reference) by beam splitter or fiber optic coupler 210 a. Insome embodiments, the beam splitter or fiber optic coupler 210 acomprises a 50/50 coupler. A first arm denoted 1 in FIG. 2A comprisesthe sample beam of the interferometer, whereas source laser beam 204 ispropagated through optical resonator 202. A second arm denoted 2 in FIG.2A is the reference beam of the interferometer, wherein the second armcomprises a replica of source laser beam 204 which has not beenpropagated through optical resonator 204. The propagated sample andreference beams may then be directed to a second splitter/combiner 210 bat an output of the resonator, where the two light beams combine and theinterference patterns are measured using detector 208.

In some embodiments, the beam splitter or fiber optic coupler 210 adivides the power of the optical beam of a CW tunable laser source, forexample, an Apex AP3350A.

In some embodiments, system 200 further comprises a balancedphoto-detector (BPD) 208, which may have, e.g., a typical common moderejection ratio (CMRR) of 30 dB and transimpedance gain of 100 kV/A. Insome embodiments, the BPD 208 provides high rejection to the intensityfluctuations of the laser source.

In some embodiments, the MZI may further include, e.g., a tunableoptical delay line, used to manually control the optical path difference(OPD) between the MZI sample and reference arms 1 and 2, respectively.In some embodiments, the MZI may further include a fiber stretcherand/or a stabilization circuit configured to lock the phases of the MZIarms to quadrature.

FIG. 2B illustrates another exemplary embodiment of a system 220 forultrasound detection, in accordance with some embodiments. System 220may have the same or similar components as system 200 in FIG. 2A.However, in some embodiments, system 220 includes interfering the outputbeam 1 with a reference beam 2 that is a replica of the propagatedsource laser beam, as propagated through optical resonator 202. In someembodiments, reference beam 2 in FIG. 2B is a time-delayed version ofpropagated sample beam 1.

Exemplary functional steps of a method for ultrasound detectionaccording to the present disclosure, using, e.g., exemplary systems 200,220, will be described below with reference to the flowchart in FIG. 3 .

In some embodiments, at step 300, an optical resonator, such asresonator 202 in FIGS. 2A-2B, may be subjected to an acoustic wave,e.g., ultrasound oscillations by an ultrasound pulse generator 206.

In some embodiments, at step 302, a detection light source, e.g., CWsource laser beam 204 in FIGS. 2A-2B, may be directed at opticalresonator 202. The output of the CW source laser beam 204 may bedescribed by the following complex field:

u _(CW)(t)=exp[−i(2πν₀ t+φ _(n)(t)],  (4)

where ν₀ is the laser frequency and φ_(n)(t) represents the phase noise.At a step 304, the detection light source laser beam 204, whose path inmarked by 1 in FIGS. 2A-2B, may be interfered with a reference opticalbeam 2. In some embodiments, the interference may be configured as partof an interferometer, e.g., a fiber-based Mach-Zehnder interferometer(MZI). For example, the light paths 1 and 2 in FIGS. 2A-2B at lightcoupler 210 a may be given by

u ₁(t)=2^(−0.5) u _(CW)(t)

and

u ₂(t)=2^(−0.5) iu _(CW)(t),

respectively, wherein path 1 represents the sample beam, e.g., in theupper arm of the MZI, and path 2 represents the reference beam, e.g., inthe lower arm of the MZI.

The sample path output of optical resonator 202 at sample MZI arm 3 andthe reference path 4 may be given by

u ₃(t)=2^(−0.5) exp[−i(2πν₀(t−ΔT ₁+φ_(n)(t−ΔT ₁)+ϕ_(US)(t)]

u ₄(t)=2^(−0.5) i exp[−i(2πν₀(t−ΔT ₂+φ_(n)(t−ΔT ₂)]  (5)

where ϕ_(US)(t) is the phase perturbation due to the ultrasoundoscillations, and ΔT₁ and ΔT₂ are the delays due to light propagation.In the expression for u₃ (t), it may be assumed that laser source 204 istuned to the center of the resonance notch of resonator 202, where T=1.The delay ΔT₁ represents the propagation time over the fiber of thesample path 1, in accordance with some embodiments, in addition to groupdelay of the optical resonator 202 at Δk=0, whereas ΔT₂ includes thepropagation delay through the fibers, delay line, and fiber stretcher ofthe reference path 2 in.

The two path may then be combined by second coupler 210 b, wherein theoutputs 3 and 4, u₃(t) and u₄(t), are interfered, leading to thefollowing fields:

u ₅(t)=0.5 exp[−i(2πν₀(t−ΔT ₁+φ_(n)(t+ΔT ₁)+ϕ_(US)(t)]−0.5exp[−i(2πν₀(t−ΔT ₂+φ_(n)(t−ΔT ₂)],

u ₆(t)=0.5 exp[−i(2πν₀(t−ΔT ₁+φ_(n)(t−ΔT ₁)+ϕ_(US)(t)]+0.5exp[−i(2πν₀(t−ΔT ₂+φ_(n)(t−ΔT ₂)].  (6)

At step 306, the power of the interference may be monitored using, e.g.,photodetector 208. Assuming that the MZI is locked to quadrature, i.e.ν₀(T₁−T₂)=N+1/4, where N is an integer, the power measurement at points5 and 6 is given by:

P ₅(t)=0.5{1+sin[ϕ_(US)(t)+φ_(n)(t−ΔT ₁)−φ_(n)(t−ΔT ₂)]}

P ₆(t)=0.5{1−sin[ϕ_(US)(t)+φ_(n)(t−ΔT ₁)−φ_(n)(t−ΔT ₂)]}.  (7)

At step 308, the transients of the optical beams at the output from thephotodetector signal may be derived. Accordingly, assuming that ϕ_(US),φ_(n)<<π during the acoustic measurement, the voltage signal at theoutput of the balanced photo-detector 208, which is proportional toP₅(t)−P₆(t), is given by

V(t)=V ₀[ϕ_(US)(t)+φ_(n)(t−ΔT ₁)−φ_(n)(t−ΔT ₂)].  (8)

Because the phase perturbations are small, the phase signal due toultrasound is given by ϕ_(US)(t)=δk_(US)(t)/α, where δk_(US)(t)represents the ultrasound-induced shift of the resonance. Accordingly,the phase noise in the measurement may be theoretically eliminated ifthe delay between the two MZI arms is identical, i.e. T₁=T₂, withoutaffecting the strength of the signal. ϕ_(US)(t).

Finally, at step 310, the acoustic signal may be derived from theoptical phase transients measured at step 308.

In some embodiments, the method comprises balancing out the arms of theinterferometer. In some embodiments, balancing the arms of theinterferometer compensates for the interferometric phase-to-intensitynoise contribution. In some embodiments, phase variations produced bymechanical or thermal fluctuations can be cancelled by the stabilizationsystem.

In some embodiments, the exemplary system as depicted by FIGS. 2A-2Bcomprises a flat acoustic PZT transducer. In some embodiments, thesystem comprises a pulse generator. In some embodiments, the PZTtransducer and the pulse generator are coupled such that squarewaveultrasound pulses are produced. In some embodiments, the pulses comprisea duration of 10 us.

In some embodiments, the method comprises producing two interferometricsignals, in counter-phase, that are detected by the BPD 208. In someembodiments, the output of the BPD 208 comprises a voltage signalproportional to the difference of the detected interferograms:

V ₀(t)=V ₀ cos[Δϕ(t)+δ(t)]+n(t)

wherein in the equation above, V₀ is the voltage amplitude, which isproportional to the fringe contrast, Δϕ(t) contains the informationabout the ultrasound signal to be recovered, δ(t) includes any randomphase fluctuations between the arms of the interferometer, and n(t) isthe voltage noise from the electronics.

In some embodiments, and in order to maximize the sensitivity of theinterferometer, a proportional-integral stabilization circuit drives apiezoelectric fiber-stretcher, locking and stabilizing δ(t) to aconstant value of π/2. In some embodiments, the stabilization andlocking system comprises a bandwidth of 3 kHz, which enablescompensation for temperature variations and low-frequency mechanicalvibrations. Under these conditions, and at weak acoustic perturbations,the output signal from the detector changes linearly with Δϕ(t).Finally, in some embodiments, V₀(t) is monitored by an oscilloscope andprocessed by a computer.

In some embodiments, method allows the decoupling between the laserphase and intensity noise. In some embodiments, if the laser is tuned atthe resonance of the grating, phase-noise do not produce intensityfluctuations of the transmitted field. Therefore, in some embodiments,laser phase-noise are converted into intensity noise due to theinterference with the reference field. Consequently, the dominantprocesses determining the noise in the method are the relative intensitynoise (RIN) and the conversion of phase noise into intensity noise. Insome embodiments, the contribution of the phase noise can be greatlyreduced by matching the OPD between the arms of the interferometer. Insome embodiments, the contribution can be reduced due to an improvementof the SNR caused by the minimization of the phase-to-intensity noiseconversion and the maximization of the contrast of the interferometricsignal.

EXPERIMENTAL RESULTS

FIGS. 4A-C illustrate experimental results using the present method andan intensity-based technique, e.g., techniques comprising a linearamplitude response. In the intensity measurements, the laser source wastuned to the maximum slope of the resonator, whereas in present method,the OPD was set to zero (T₁=T₂) for maximum SNR.

The laser power was 0.1 mW in both cases, which was selected so as notto saturate the photodetector. As can be seen in FIG. 4 , the twosignals are almost identical, whereas the SNR obtained with presentmethod is 8 times lower.

To evaluate the effect of the OPD on the SNR of the present method, themeasurement was repeated for different values of OPDs over a span of 30cm. The noise is determined by calculating the standard deviation of thesignal before the arrival of the ultrasound pulse (t<10 μs in FIG. 4 )over a bandwidth of 4 MHz (i.e., the bandwidth of the photodetector).

The output noise density of the present method as a function of the OPDis shown in FIG. 4B, as expressed both in terms of noise and SNR gain incomparison to intensity-based measurements. FIG. 4B also shows the noisedensity of the electronics (dark-current noise). As expected, when themagnitude of the OPD is increased, the noise cancellation obtained withthe present method diminishes. Additionally, it is noted that when theOPD was set to zero, the noise in this measurement was due to the darkcurrent of the photo-detector.

In the final measurement, the SNR of the present method was tested for azero OPD at different power levels. Since the trans-impedance amplifierin the balanced photo-detector amplifies the differential signal fromthe two photo-diodes, rather than each individual signal, eachphoto-diode may be illuminated with powers higher than 0.1 mW withoutleading to signal saturation.

FIG. 4C shows the SNR gain of the present method for different powerlevels with respect to the maximum SNR of intensity-based measurementsfor the same acoustic signal, which was reached at a power ofapproximately 10 μW. The figure is presented as a log-log plot andreveals three regimes of SNR. For powers up to approximately 0.1 mW, theSNR grows linearly with the power, i.e. a slope of approximately 1 inthe log-log plot, indicating that the main noise source is from the darkcurrent of the photo-detector. For powers between approximately 0.1 mWand 1 mW, the slope of the curve is approximately half, indicatingshot-noise limited detection in which the SNR grows as the square rootof the power. Finally, for powers above 1 mW, the SNR gains diminishedfurther, reaching full saturation at 2 mW.

In some embodiments, the present method more robust than theintensity-based technique due to at least one of the laser phase noisecompensation and the balanced photodetection, which provides asignificant enhancement of the SNR without a significant addedcomplexity to the system. Additionally, in some embodiments, the presentmethod can be adapted to schemes with high dynamic range andmultiplexing capabilities, such as pulse interferometry.

What is claimed is:
 1. A system comprising: an optical resonatorconfigured to be impinged by acoustic waves; a continuous-wave sourcelaser beam directed to said optical resonator, wherein said source laserbeam is propagated through said optical resonator, thereby generating apropagated laser beam; and an interferometer configured for detecting asignal associated with said acoustic waves by monitoring transients inthe optical phase in said propagated laser beam, wherein said transientsare indicative of a waveform of said acoustic waves, wherein saidmonitoring comprises interfering said propagated laser beam with areference beam.
 2. The system of claim 1, wherein said reference beamcomprises a replica of said source laser beam.
 3. The system of claim 1,wherein said propagated laser beam and said reference laser beam have anoptical path difference equal to zero at a wavelength of said sourcelaser beam, and wherein said reference beam comprises a time-delayedversion of said propagated laser beam.
 4. The system of claim 1, whereinsaid optical resonator is selected from the group consisting of: πphase-shifted Bragg grating (π-BG), Fabry-Perot cavity, and optical-ringresonator.
 5. The system of claim 1, wherein said laser beam is tuned toa center of a transmission notch of said optical resonator.
 6. Thesystem of claim 1, wherein said interferometer is a Mach-Zehnderinterferometer (MZI).
 7. The system of claim 1, wherein said monitoringfurther comprises measuring an optical power transmission in saidpropagated laser beam and said reference laser beam, wherein saidoptical power transmission is indicative of said transients.
 8. Thesystem of claim 7, wherein said measuring of said optical powertransmission is performed by at least one balanced photo-detector. 9.The system of claim 1, wherein said acoustic waves are ultrasoundacoustic waves, and wherein said ultrasound acoustic waves are generatedopto-acoustically via the transformation of a modulated optical beaminto acoustic waves.
 10. The system of claim 9, wherein the modulatedoptical beam consists of optical pulses.
 11. A method comprising:directing a continuous-wave source laser beam to an optical resonatorthat is impinged by acoustic waves, wherein said source laser beam ispropagated through said optical resonator, thereby generating apropagated laser beam; and detecting a signal associated with saidacoustic waves by monitoring, using an interferometer, transients in theoptical phase in said propagated laser beam, wherein said transients areindicative of a waveform of said acoustic waves, wherein said monitoringcomprises interfering said propagated laser beam with a reference beam.12. The method of claim 11, wherein said reference beam comprises areplica of said source laser beam.
 13. The method of claim 11, whereinsaid propagated laser beam and said reference laser beam have an opticalpath difference equal to zero at a wavelength of said source laser beam,and wherein said reference beam comprises a time-delayed version of saidpropagated laser beam.
 14. The method of claim 11, wherein said opticalresonator is selected from the group consisting of: π phase-shiftedBragg grating (π-BG), Fabry-Perot cavity, and optical-ring resonator.15. The method of claim 11, wherein said laser beam is tuned to a centerof a transmission notch of said optical resonator.
 16. The method ofclaim 11, wherein said interferometer is a Mach-Zehnder interferometer(MZI).
 17. The method of claim 11, wherein said monitoring furthercomprises measuring an optical power transmission in said propagatedlaser beam and said reference laser beam, wherein said optical powertransmission is indicative of said transients.
 18. The method of claim17, wherein said measuring of said optical power transmission isperformed by at least one balanced photo-detector.
 19. The method ofclaim 11, wherein said acoustic waves are ultrasound acoustic waves. 20.The method of claim 19, wherein said ultrasound acoustic waves aregenerated opto-acoustically via the transformation of a modulatedoptical beam into acoustic waves, and wherein said modulated opticalbeam comprises optical pulses.